Thanks for the replies!
Yes, she pays CLASS A PRSI. Seems from her payslips that her Gross has remained the same when on Maternity, which is ok - but there's no mention on the payslip that this Gross amount includes Maternity Benefit payment, so all of the deductions (USC, PRD, PAYE) have remained the same! Will have to chase up that Balancing Statement at the end of the year, as it looks like she might be overpaying.
Will she get the Pension Related Deduction that was taken on the Maternity Benefit portion refunded from Employer or Revenue?